How can marginalised communities organise a project to yield significant social change? This thesis theorises the resources which enable such community organisation to work. Participation, empowerment and conscientisation are understood, not through a logic of quantity which creates linear dimensions, but through a logic of concrete qualities. A pragmatist approach is taken, to define our constructs in terms of the actions being undertaken by participants, within specific, qualitatively distinctive domains. Activity theory is used to theorise participation as a process of collective activity, which is supported by shared rules, a division of labour and shared goals, and which is challenged by divergences of interest. A community case study of the Sonagachi Project, a successful HIV prevention project run by sex workers in Kolkata (India), is used to investigate participation. The case study is based on interviews and group discussions with sex workers and Project workers (sex workers employed by the Project), and observation of the daily activities of the Project. Sex workers relate to the Project as a source of support in solving their individual problems, gaining new powers, but not acting as collectivity members. Project workers are constituted as collectivity members, whose action interlocks with that of their colleagues, through participating in the politicising discourse of the Project, which states that sex workers should be granted “workers’ rights”, and through learning the rules of participation in meetings and the hierarchical division of labour. To be allowed to operate, the Project has to carefully adjust to local power relations, with madams, political parties, and funding agencies, in collaborative-adversarial relationships. In conclusion, the scope of participation is defined as producing significant, yet circumscribed, local change. To intervene in a fractured community is a political process in which the provision of new resources is both necessary and potentially divisive.